I believe the original idea was mainly focused around making software developer’s lives easier, by (as you say) making versions of linux os’s with known, static, unchanging, immutable, standard system configurations, which could not be altered by the user.
Again, I think the main idea here was basically focused around making either new distros, or sort of locked down versions of distros such that they could be deployed in I guess an enterprise or some kind of group environment, so as to greatly streamline the process of software development.
You say ‘can receive updates’, and I am not sure if you mean higher level app updates, or lower level system updates.
When people attempted to evangelize this immutable concept to me a few years back, they would say the whole point is that lower level updates on which many other things depend do not happen, that you would have to do basically a completely new install of a new immutable version when one comes out.
Kind of like ‘more stable than stable.’
But… judging by other comments here, it would appear that this concept of immutable has uh… changed, lol.
Also, for what its worth, the Steam Deck OS is basically just a standardized version of Arch, upon which sits the whole nifty Steam Deck UI and Controller mappings, with the system set to read only by default.
You can change this and then do whatever the hell you want, if you so please.
I think its just one line in the terminal after you have set up your user as a system admin.